Over the course of the space race, why wasn't more attention given towards Venus or Mercury?

[u/TheHondoGod]

Especially compared to Mars or the outer planets. I know there were proves that went to the inner planets, but it seems like so much of the focus was outward.

[u/Downtown-Act-590]

Let us start with stating that part of the premise of this question is not really true.

There was a lot of attention to Venus given during the Space Race. Actually, more than to Mars. In the first 15 years of interplanetary flight (cca. 1960-1975), the Soviets missed only one launch window to Venus and often launched more than one probe during a single one [1]. Launch window is a situation where the other planet can be reached with a smallest amount of energy and for Venus they appear every 19 months. So, that is a quite a lot of launches!

While the US was much less active towards Venus than the Soviets, it still sent multiple Mariners, the Pioneer Venus probes (which even had a lander part) and also Magellan towards the planet. Perhaps, this feeling of Venus being neglected is caused by the combination of Venus being the Soviet domain with the fact that no-one was much interested in Venus after the Space Race ended in a sharp contrast to Mars. But we shouldn't speculate too much.

What is true though, is that Mercury didn't get nearly the same amount of attention as Venus or Mars. And that is because Mercury is a completely different animal altogether when it comes to exploration.

Mercury is in an orbit very close to the Sun and as a result any object on Mercury (or orbiting it) will have a quite low orbital energy with regards to the Sun. Much lower than it would have on Earth. If we want to go to Mercury, we need to lose a lot of energy!

This amount of energy to lose will decrease, if we send a light spacecraft. But there are technical limits to how light we can go. And we have only two ways to get rid of this energy (or at least we had during the Space Race), bring a lot of fuel or/and do so-called gravity assist maneuvers. We can also vary these energy requirements by limiting scope of our mission. We can either choose to just fly by, orbit or land on the planet. Each further step brings us more knowledge about the planet, but also requires more energy to accomplish.

Flying to Mercury directly results in ridiculously high energy requirements, one has to use a gravity assist. This complicates things significantly as you have to wait for the planets to be in the right configuration, which may happen rather rarely. The only Space Race Mercury mission was Mariner 10 and it used a gravity assist around Venus to achieve a mere flyby. In [2] it is outlined that an orbiter mission around Mercury with Venus gravity assist would require a Saturn V-sized launch vehicle. That was a very difficult requirement in the climate of relative austerity of the mid-1970s as [2] also notes.

Even though it was possible to reach Mercury using a smaller launch vehicle through a complicated series of gravity assists, the opportunities in the 1980s were relatively scarce [2]. Despite the continued studies of the orbiter problem, the NASA mission did not materialize. One factor could be the very drastic thermal environment close to the surface of the planet, which required rather novel thermal control systems as [3] suggests. Another notable fact is that NASA was already aware of advanced concepts like solar sailing and solar electric propulsion in the 1970s and knew that after these technologies mature, missions to Mercury will be much more doable [4] (which eventualy happened quite recently with missions like BepiColombo). We again should not speculate too much though.

With regards to USSR and Mercury, someone else will have to give you the answer.

[1] Ball A, Garry J, Lorenz R, Kerzhanovich V. Planetary Landers and Entry Probes. 2007

[2] Hollenbeck G R, Study of Ballistic Mode Mercury Orbiter Missions. 1973

[3] New concepts for Mercury orbiter missions. 1978

[4] Friedlander A L, Goldfinch H. Mercury orbiter transport study. 1977

[u/TheHondoGod → u/Downtown-Act-590]

Thank you! Really appreciate it.